Cold cathode ionisation vacuum measuring gauges, sometimes referred to as “Penning” gauges, generally comprise an anode and one (or more) cathodes with a large potential difference applied between the anode and the cathode(s) and a magnet that applies a substantial magnetic field in the area between the electrodes. The potential difference between the anode and cathode can be in the range 2 to 5 kV and the magnetic field can be generated by a permanent or non-permanent magnet. The anode and cathode(s) are held in a predetermined configuration relative to each other, which isolates the electrodes within the gauge from the atmosphere outside.
Cold cathode ionisation gauges rely for their operation on ionising the atoms and molecules of the gas whose pressure is being measured by generating a plasma within the gauge. Electrons can be emitted by the cathode(s) and accelerated towards the anode by the electric field. Collisions between the electrons and gas molecules as the electrons move towards the anode form positive ions that are attracted by the cathode(s) to produce an ion current in an external circuit. The action of the magnetic field causes the electrons to adopt a very long, non-linear, trajectory prior to striking the anode. This increases the likelihood of an electron colliding with and ionising gas molecules before it is captured by the anode. The magnitude of the ion current is related to the number density of the gas at a given temperature and therefore to the level of vacuum.
To initiate an ion discharge, some free electrons must be present within the gauge envelope; a certain number of free electrons are likely to arise due to random events. The free electrons are accelerated towards the anode by the applied potential difference. There is a probability that some will collide with residual gas molecules, producing ionisation of the molecules and the release of further electrons. The newly released electrons will be similarly accelerated and may produce further gas collisions, ions and electrons. The ions arising from electron collisions will be accelerated towards the cathode and, when they strike it may cause the release of further electrons by secondary emission processes.
For an ion discharge to be built up and sustained, the rate at which new free electrons are generated, by collisions within the gas and secondary emission, must initially exceed the rate at which electrons are captured by the anode. Unless free electrons are produced at a greater rate than the capture rate, ion discharge will fail to establish itself.
When the ion discharge is fully established it stabilises at a level such that the flows of ions and electrons to the cathode and anode respectively reach a value, which is dependent on the number density of gas molecules within the discharge chamber of the gauge. Hence the suitability of the ion current as a measure of the gas pressure.
When a cold cathode ionisation vacuum gauge is switched on at a very low pressures, for example less than 1×10−5 mbar, it may fail to “strike” (i.e. an ion discharge may fail to establish) for a considerable time. At low pressures the chance of randomly occurring free electrons is reduced, as is the chance of such electrons making numerous collisions with residual gas molecules. The result is the gauge may take several minutes or even hours to strike because the probability of an ionising event occurring is reduced due to low gas density. This problem may be accentuated if, in service, the electrode structure becomes coated with contaminating layers. Contaminating layers can build up in gauges used in industrial high vacuum systems where many sources of contamination, including organic vapours, enter the gauge head from the pumping system. Contaminant layers formed by adsorption onto the electrode surfaces may affect their secondary emission characteristics and can be particularly effective in inhibiting the proper establishment of ion discharge when the gauge is switched on.